Demo board

Small PCBs (is there a better term than "small PCB" for the sorts of PCBs that are specifically designed to be hacked and modified -- they have pins that are perfectly aligned to be plugged into a solderless breadboard, or they have a prototyping area on-board?)

Rather than buying a raw microcontroller, many people buy a small PCB that includes the microcontroller and other essential Popular Parts on a tiny little microcontroller development board. Many such boards are demo board, easy to plug into your solderless breadboard (or your custom PCB) that has just a few more things that make your project special.

Later I learn:
"A plugin is a small electronic plug-in module capable of being plugged directly into a breadboard, that fulfills a (usually simple) function."
I also discover some people call these things "breadboardable" or "DIP-like"[1][2] or "breadboard compatible"[3].

further reading

Form factor comparisons and reviews

What are standard interfaces between a "CPU board" and a "I/O board"?
If none of the standard interfaces are quite what I want,
what are good things to keep in mind when designing a new interface?

"Rethinking the Arduino hardware interface" describes the differences between the "JeeNode form factor and I/O connectors" and the "Arduino Shield form factor and I/O connectors" (both systems are fully compatible with the standard Arduino IDE software).

Demo boards, in no particular order

PIC based boards

Ready to use daughter boards based on PIC microcontrollers. Each model is designed to meet different price/performance requirements with the smallest possible footprint

OOPic: The Object-Oriented PIC. The "ooPIC-S" board is a PCB with a large prototyping area and a PIC16F877. The "ooPIC-C" is a tiny stamp-like PCB that plugs into a solderless breadboard. Both hold a PIC16F877, a Flash memory chip, 3 LEDs, a voltage regulator, and a RS-232 tranciever). Both run the "ooPIC III+" firmware.

by Brian Schmalz
...
"The UBW board ... contains a Microchip PIC USB-capable microcontroller, headers to bring out all of the PICs signal lines (to a breadboard for example) ... costs about $15-$20 to build and is powered from the USB connection. ... All tools used to design this project are free, and the design is open to anyone to use for whatever they wish. Please build your own and improve upon it! If you don't want to build your own, you can buy them assembled and tested ... SparkFun hosts a USB forum where the UBW is a common discussion topic."

Dontronics sells a variety of PIC-oriented proto boards and development boards (as well as development boards for several other popular microprocessors).

iRX Prototyping Board: a simple but malleable prototyping board for PIC based projects. It features the EEPROM programmable PIC16F84A, an MAX233 for RS232 port and infrared I/O. About half of the board is an empty prototyping area. Compare: Atmex.

Sirius microSystems has several "easily hackable" development boards, mostly based on Microchip PICmicro. Also posts circuits and schematic diagrams under a "Creative Commons Attribution-Noncommercial-Share Alike 3.0 License", making them "open hardware".

microEngineering Labs, Inc. specializes in development tools for Microchip PIC® microcontrollers." Lots of completely empty boards designed to be stuffed with various PICmicro chips and associated components ... such as the $19.95 PICPROTO-USB -- compatible with 28 and 40-pin devices, this board is equipped with additional USB circuitry and connector for use with the USB-equipped PICmicros like the 16C745 and 18F4550.

Arduino boards

There's a huge number of AVR-based Arduino-like boards at Arduino Links, most of which are breadboard-friendly.

ARM boards =

ARM links - there are a growing number of Arduino-like embedded ARM boards, offering more CPU power than Arduino but many of the same capabilities.

Other demo boards

AMPS is a modular prototyping system for microcontrollers from Technological Arts, focused primarily on HC11, HC12, S12 and S12X chips from Freescale. It's like lego for microcontrollers because all the connectors and form-factors of the "Adapt" cards are standardized and interchangeable, enabling you to arrange them in various ways. (http://www.technologicalarts.ca/catalog/index.php?cPath=50)

Bread Board Power Supply -- a tiny little board that fits between a wall-wart and plugs into a solderless breadboard to supply regulated power. It accepts a wide variety of wall-warts because its full-wave rectifier and LM317 handle a wide range of AC and DC power supply, and its screw terminal headers let you plug in the wires.

A huge list of Makezine kits, a few of which ("Daisy MP3 player", some of the RepRap projects, "The IR-ritator", etc.) use PICmicros, a few others that use Atmel AVRs (TV-B-Gone, etc.), one that uses a Freescale processor ...

SpinStudio, a system designed for development using Parallax's Propeller(TM) Microcontroller.

AVR Atmex: An entry-level Atmel Experimentation System. less than $10; includes Atmel ATtiny2313; a DB-9 and MAX232 for serial communication and downloading new programs (using a bootloader); LED. About half of the board is an empty prototyping area. Meant to be better than the iRX board.

...

"NanoCore12", much like the BASIC Stamp but using a Motorola/Freescale HCS12 instruction set http://www.nanocore12.com/ from Technological Arts. It has a free object-based programming environment for Windows, called nqBASIC (http://www.nqBASIC.com)

Micromint http://www.micromint.com/ sells little boards with DIP pinouts (and so can plug into protoboards) based on ARM7, Atmel AVR, and 8051 cores ...

...

"The uCsimm module is a microcontroller module built specifically for the uClinux Operating System. It stands an inch high, with a standard 30-pin SIMM form factor. ... The uCsimm is driven by a Motorola DragonBall 68EZ328 processor, and comes well equiped with 2 MB of FLASH and 8Mb of DRAM. We have also included a 10Base-T ethernet and RS 232 high-speed serial." http://uclinux.org/ucsimm/

"Picotux was the smallest computer running Linux in the world. ... 35 mm × 19 mm × 19 mm and just barely larger than [its] RJ45 connector" and its DB9 serial connector. "... Two communication interfaces are provided, 10/100 Mbit/s half/full duplex Ethernet and a serial port with up to 230.400 bit/s. Five additional lines can be used for either general input/output or serial handshaking. ... 55 MHz 32-bit ARM7 Netsilicon NS7520 processor, with 2 MB of Flash Memory (750 KB of which contains the OS) and 8 MB SDRAM Memory. The operating system is µClinux" -- Wikipedia: Picotux

AVR StAVeR-24M32 "a microcontroller module based on an Atmel ATmega32 in a small DIP24 package with a standard pin out."

Copper (AVR32 Stick) "Copper is an Atmel AVR32 UC3B1256 mounted on a DIP32 breadboard friendly module, giving you the smallest AVR32 development board available (and one of the cheapest!). Features integrated voltage regulator, USB connector, and in-system programming via the USB port. ... The board design and schematics are available under the Creative Commons Attribution Share-Alike 3.0 license. ... 256KB of flash, 32KB of RAM, on board USB, up to 60MHz CPU clock ..."